1. The importance of flatness

Pavement flatness is an important indicator for assessing the quality of asphalt pavement. The levelness of the road surface is not only directly related to the safety of the vehicle, comfort, and the running performance of the vehicle and the economic operation, but also the unevenness of the road surface, causing additional vibration of the vehicle. The effect, in turn, exerts an impact on the road surface, thereby exacerbating the damage to the road surface. In road engineering, flatness is one of many road technical indicators. However, for the use of roads, flatness is one of the factors that affect road use. Here, the author combines the construction experience of the CDE-21 standard of the Lian-Xu Expressway to make a brief analysis of the control of the construction of the high-grade asphalt pavement, so that attention can be paid to the construction of the asphalt pavement.

2. Factors affecting the flatness of asphalt pavement

In asphalt pavement construction, there are five main factors that affect the flatness of asphalt pavement:

2.1 The influence of the flatness of the base layer on the flatness of the surface layer

The pavement structure of a road is a layered structural system. It is generally composed of a surface layer, a base layer, a base layer, and a cushion. In the pavement structure layer, the flatness of the bottom layer directly affects the flatness of the top layer, so ensuring the flatness of the base layer is a prerequisite for ensuring the flatness of the asphalt surface.

2.2 Influence of Asphalt Mixture

Asphalt mixture on high-grade pavements usually adopts three types: fine-grained, medium-grained, and coarse-grained. In the construction, when the asphalt mixture is mixed into the large-sized stones and enters into the paver, it will have an adverse effect on the paving and roller compaction of the machinery, especially for road surface flatness. After the screed of the paver reaches the super-size gravel, the controlled work surface will change. In the subsequent mechanical roller compaction, the strength of the gravel is relatively high and it is not easy to crush, thereby affecting the flatness of the entire surface layer. The surface layer has a convex groove in this area.

2.3 The impact of the datum

Pavers used in high-grade highways all have an automatic leveling function. Using an automatic leveling device requires an accurate datum. The commonly used datum control is the baseline wire method, the sliding method and the average beam method.

2.4 Paving Operation Impact

The operation of the paver and its own adjustments have a great influence on the paving quality. The following points deal with several issues that should be noted in the use of pavers during construction.

(1) Changes in the speed of the paver will result in changes in paving thickness. The principle of selecting the speed of the paver is to ensure continuous operation of the paver. The reasonable speed can be obtained according to the formula supply capacity, paving thickness and width as follows: V=100QC/60ρWT

Q─mixer output t/h

V - paving speed m/min

T─Pad thickness after compaction cm

C—The efficiency coefficient is determined according to the supporting conditions such as material supply and transportation capacity. Should be 0.6 to 0.8

W-Paving width m

ρ─Asphalt mixture compacted density t/m3

(2) Adjustment and maintenance of the screed. The bottom surface of the screed is worn or severely deformed, and the surface layer is prone to cracks and grooves when paving, so it should be replaced in time.

(3) Excessive tightness of track-type paver track will cause a pulse at the paving speed, making the paving surface form a see-saw.

2.5 Impact of roller compaction

Compaction is the last step in the construction of asphalt pavements, and good road quality ultimately needs to be reflected through roller compaction. The main contents of the compaction work include the selection and combination of roller compaction machinery, the determination of compaction temperature, speed, and compaction methods, and the compaction of special sections (curves, steep slopes, etc.). In normal construction, the following matters should be noted in the roller compaction operation: (1) During the operation of the roller, in the smooth road section, the drive wheel is close to the paver, which can reduce ripples and thermal cracks. (2) Control of rolling temperature and rolling variables. If the rolling temperature is too high, it will easily cause the transfer of the paving layer, it will be difficult to form a stable structure at the final pressure, the temperature will be too low, the mixture will appear stiff, the frictional resistance between the aggregates will increase, and the compactness and flatness of the mixture will increase. It is not easy to reach. (3) The variability of the rolling speed and the excessive start of the roller. Reversing, braking and reversing all affect the flatness of the road surface.

3, measures to improve the smoothness of asphalt concrete pavement

3.1 Mixing ratio adjustment:

In the construction, the grain size of the mineral material should be strictly controlled so that the maximum grain size is less than half of the paving thickness.

3.2 Paving Control:

As modified asphalt is used on the upper layers of highways, non-contact leveling beams are recommended on the upper layer.

Reasonably spread the speed so as to ensure the continuous discharge of mixing and discharging. The paver travel speed is 2～4m/min. At the beginning of construction, the speed of the pavers is adjusted according to the number of vehicles to be unloaded and the mixing speed. When the paver is left behind with less than 4 vehicles, the paving is fine-tuned. Speed ​​to ensure continuity of paving operations.

3.3 Rolling control

Special person to supervise the roller compaction process: The roller compaction process is under the command of a special person. The initial pressure, recompression and final pressure sections are marked with different colored flags to avoid overpressure and leakage pressure. When roller compaction starts, the start speed of the roller is slow, and it stops automatically when turning back. It is forbidden to brake and stop urgently to avoid the change of mixture.

The person inspects the road surface smoothness: After the repressurization is over, the quality inspector promptly checks the flatness of the road just rolled over the road with a 3m ruler, and makes a corresponding mark. Before the final pressure, the roller according to the mark of the inspector, Take corresponding measures. If the lateral flatness is relatively large, then the lateral vibration is 2 to 3 times; if there is a problem in the vertical flatness, then the strong vibration in the longitudinal direction is 2 to 3 times, so that the flatness can reach a high level.

Strictly control the rolling temperature: The control of the rolling temperature is an important step in the rolling process and is the primary factor in improving the flatness and compaction. The higher the temperature of the mixture, the easier it is to compact and it also helps to improve the flatness. If the temperature is low, the internal friction of the mixture will increase, and it will be difficult to achieve an uneven compaction flatness index.

The temperature of the asphalt concrete from the production to the completion of the paving process is measured by a person, especially from the paving to the rolling stage, and the temperature of the mixture is recorded in detail, which provides a reliable basis for the rolling speed and time.

3.4 Seam Processing

In the joints of the previous construction, a 3m ruler was used to perform the inspection in advance, and the section where the flatness did not meet the requirements was firmly cut off and was never soft. Before paving, press the hot plate to the original paved surface and fully preheat it. At the joint, the initial pressure direction of the drum roller should be at an angle of 45 degrees with the direction of the route, so that the mixture can be dispersed so that it is not on the same plane. There are no horizontal construction joints in the downhill section, in the narrow road section and in the obstacle sections on both sides of the road. The joints are only installed in the flat section and the uphill section.

4 Conclusion

The construction of asphalt concrete pavements on expressways is a technical project with a wide range of technologies. Modern construction machinery and mature construction techniques are necessary quality assurance measures. At the same time, it is necessary to have an orderly process organization, a strict quality assurance system, high-quality operators and management personnel, and to start from all aspects of construction. Assortment of asphalt mixture, transportation, paving, roller compaction and other aspects are strictly controlled, and only when these aspects are effectively combined can high-level pavement be built.

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